Electron microscopes use a beam of accelerated electrons, which pass through or are deflected by a sample to provide an electron image and/or diffraction pattern of the sample. To provide a record of these images and/or diffraction patterns, at least a portion of the kinetic energy of the electrons is converted into another form of energy which can be measured and permanently stored. For example, light images are generated by impinging the electrons onto scintillator materials (e.g., phosphors). In this application, “scintillator” and “phosphor” are used interchangeably to mean a material that emits light when excited by ionizing radiation (electron, gamma ray, etc.) As shown in FIG. 1, a scintillator 10 forms light images and/or patterns that may be captured on a two-dimensional imaging sensor 20 via a fiber array 30. The imaging sensor may be a charge coupled device (CCD) or a CMOS imaging detector. The output from the imaging sensor may be read as an analog signal, converted to a digital format by an analog to digital converter, and displayed on a video monitor and/or stored permanently.
Once an accelerated electron enters the solid volume of a detector (scintillator) it starts to lose energy to the solid. The rate of energy loss depends on the initial energy of the electron and the solid material through which it is traveling. The electron is also scattered randomly by the fields surrounding the atoms of the detector in a manner which alters the electron's direction or path of travel. The result is that a series of accelerated electrons of the same initial energy, entering the solid detector at a specific point, will generate a set of paths which together fill a region of space resembling a pear-shaped cloud, see FIGS. 2A-2E, which show simulated paths of electrons at 60 keV, 100 keV, 160 keV, 200 keV and 300 keV, respectively, as they scatter in the scintillator and continue scattering in the substrate. As shown, the beam scatters through the high-density scintillator layer between the two white lines at the top. The electron then proceeds through the lower density fiber optic substrate, and the scattering density is reduced (e.g. the electron mean free path is increased in the fiber optic substrate as compared to the scintillator layer). The relevant information is contained in the light produced inside the bright region in the scintillator just below where the electron impinges. This region must be kept small in all three dimensions. The volume of the scatter region can be defined as the envelope of all possible paths and is termed the interaction volume of the electron beam in the detector. The energy of the electron beam and the average atomic number density (Z density) of the detector material (i.e. the phosphor) together determine the electron path's average behavior and thus the size and shape of the interaction volume.
Higher electron energies cause the interaction volume to be larger, while denser materials in the detector will cause it to be smaller. The interaction of high energy electrons with the volume of the solid material of the detector generates spreading and noise which constitute primary limitations on the amount of spatial and intensity information obtainable from the incident electron image.
One approach to reducing interaction volume is to make the scintillator as thin as possible. A disadvantage of this approach is that only a small fraction of each electron's energy is utilized in the scintillator, and that fraction grows smaller with increased energy, limiting sensitivity. This can be seen in FIG. 3, which is a simulation of deposited energy vs. thickness and indicates that the lower energies (<120 keV) approximately stop energy deposition after 12 μm to 20 μm. Note that a substantial portion of the energy is deposited in the top 10 μm of the thin film. As shown in FIGS. 2A-E, the width of the deposition in the image plane grows as the further into the scintillator the energy is deposited—hence the need for thin and dense scintillators.
Increasing scintillator thickness increases sensitivity, but also increases scattering and degrades resolution on the imaging detector. The density of the scintillator material becomes important in the case of a thin film. For a given thickness a higher density material will interact more with the electron beam than a lower density material, so the interaction volume is reduced while the resulting signal is increased. The final resolution of any sensor imaging device recording these images and patterns is determined by the combined effect of 1) scattering of the incident electrons by atoms in the scintillator material and supporting structure for the scintillator, 2) spreading and random scattering of the electron-generated photons by boundary and grain surfaces in the scintillator, 3) scintillator particle grain size and optical scatter in the film, 4) the resolution of the transfer optics from the scintillator to the sensor imaging device, and 5) the intrinsic resolution of the sensor imaging device.
Phosphors excited by electron beams typically have a light output behavior over time after the end of the electron beam exposure period with at least two recognizable parts. The first part is a fast-decaying, high intensity, portion over a primary decay time and the second part is a very slowly-decaying intensity tail, often called afterglow. For example, in a scintillator such as the often-used P-46, light intensity drops to a factor of 1/e (˜37%) of the initial level within 1 μs primary decay time, but not below 1% afterglow until 100 μs has elapsed. While decay times down to ˜2-3% of peak emission can be very short, below these levels the long tail decay modes predominate, and can extend decay times to 10-3000 μs for ˜1% of peak emission in phosphors typically used in TEM. The tail becomes an impediment when multiple images must be made in short time frames, as in the case of in situ imaging and Scanning Transmission Electron Microscopy, “STEM” where a focused beam is raster-scanned across a sample and the signal generated at each point is recorded and then assembled into an image. An Electron Energy Loss Spectrometer may also be used in STEM to record energy spectra for each scanned point. The speed with which the electron beam can be scanned is limited by among other things, the long temporal-response tail on the scintillator phosphor.
A spectrum image may contain data from millions of pixels so the ability of the camera to record images or spectra at fast data rates is critical. Spectrum imaging applications are reaching data rates that challenge many of the known scintillator materials. Standard scintillator materials such P20 and P43 are used because of their high conversion efficiencies. Slow decay characteristics of these materials, however, becomes problematic in high rate measurement applications, and newer materials like P46 (Gd2O2S:Tb—300 ns decay) and P47 (Y2SiO5:Ce,Tb—100 ns decay) began to be used. Although P46 and P47 have fast decay properties, as shown in FIG. 4, these materials suffer from significantly lower efficiencies than their predecessors.
Thus, there is a need for a thin scintillator having high density, high conversion efficiency and short decay time.